1. Field of the Invention
The present invention relates to a hydrocarbon oil based lubricant comprising a combination of a polar polyalkyl(meth)acrylate copolymer and an ester oil.
2. Description of the Related Art
For more than 50 years, the lubricant industry has sought efficient ways to modify the viscosity of various fluids to improve the overall lubricity of the fluid for applications in crankcase fluids, transmission fluids, gear oils, and hydraulic oils. The viscosity index (VI) of a fluid, refers to the ability for a fluid to maintain viscosity and lubricity over a specified temperature range, most often between 40° C. and 100° C. Increasing the VI of a fluid not only leads to enhanced lubrication, but also can provide additional benefits and utilities which may distinguish the overall performance of one fluid versus another. Such benefits may include reduced viscosities at colder temperatures thus improving low temperature performance and improvements in hydraulic pump efficiency for various hydraulic systems, which can ultimately lead to reduced fuel consumption.
The conventional base fluids for lubricants are mineral base oils (Groups I-III), synthetic oils such as poly alpha-olefins (Group IV) or ester oils (Group V). For the purposes of the present invention the term hydrocarbon oils will be understood to describe both mineral oils (Groups I-III) and poly alpha-olefins (Group IV). The viscosity index of these base fluids generally increases as the fluid changes from a Group I to Group V. Synthetic base fluids (Groups IV-V) are beneficial for their favorable low temperature properties and their high viscosity index.
The viscosity index of a lubricant formulation may be modified by addition of a viscosity modifier or by altering the composition of the base fluid. Viscosity modifiers may conventionally be selected from polymers such as polyolefins and polymethacrylates. Poly(alkylmethacrylates) (PAMAs) are conventionally employed as VI improvers to obtain favorable viscosity profiles in lubricating oils at high and low temperature. Chemical modification of poly(alkylmethacrylates), such as, for example, compositional modifications, molecular weight/shear stability adjustments and solvent selection may affect performance of the polymer as a VI improver in a lubricant composition.
Due to ever increasing demands on lubricants, in particular, hydrocarbon oil based lubricants, for better performance which would contribute to reduced fuel consumption and reduced frictional wear leading to increased engine or pump performance lifetime, the industry is continuously exploring new methods and technologies to improve lubricant performance and increase the VI of the lubricant formulation. The need for increasing viscosity index is important for many applications requiring lubrication, where incremental increases can result in vast improvements in performance and efficiency.
JP2007031666 describes methacrylate-based VI improvers prepared in a solvent such as ester-oil synthetic solvent, which increase the VI of ester-based synthetic fluids. The described viscosity index improvers, contain a copolymer (A) comprising alkyl(meth)acrylate(a1) selected from group consisting of C1-4 alkyl and C1-4 hydroxyalkyl(meth)acrylate ester, C11-15 alkyl(meth)acrylate ester(a2), and C16-24 alkyl(meth)acrylate ester(a3). A solvent (D) may be an aliphatic solvent, aromatic solvent or ester based synthetic oil.
JP2007031666 provides no indication that the copolymers are useful for improving the VI of hydrocarbon oil-based formulations.
JP 2006077119 reports the use of various ester oils which are used as solvents for synthetic base fluids. These ester-based synthetic fluids have benefits in low temperature viscosity, gear lubricity, and hydraulic actuation. However, there is no disclosure or suggestion of improvement in viscosity index of the final fluid.
JP 2627725 describes the synthesis of ethylene-alpha-olefin-MA based copolymers which may contain grafted side-chains and VI improvers containing the copolymers. The VI improvers are added to lubricating oils based on mineral oil, synthetics, ester-based synthetics and mixtures thereof.
U.S. Pat. No. 6,303,548 describes a lubrication oil which is a combination of a mineral basestock, a poly-alpha-olefin and a synthetic ester. A broad range of potential viscosity improvers which are prepared in a solvent is described. Potential viscosity modifiers in this crankcase application include alkyl methacrylate copolymers, olefin copolymers and poly-hydrogenated butadienes.
EP 992570 A3 describes a hydraulic lubrication oil containing one of a mineral oil, poly-alpha-olefin or ester based synthetic as a base fluid. EP 992570 A3 provides no discussion of VI benefit or notable low temperature benefit by addition of ester oil as an additive.
None of the above patents discloses or suggests that improvement in the VI of a hydrocarbon oil based lubricant can be obtained with a combination of copolymer having a polar composition and an ester oil.